Physical characteristics

Although most sigmodontines are typical mice and rats, the group exhibits several deviations from this morphotype.

A cactus mouse (Peromyscus eremicus) in the California desert, USA. (Photo by Anthony Mercieca/Photo Researchers, Inc. Reproduced by permission.)
A deer mouse (Peromyscus maniculatus) standing on lichen. (Photo by Maslowski/Photo Researchers, Inc. Reproduced by permission.)

Examples of these departures are the shrew mouse Blarino-mys, the mole mouse Geoxus, the chinchilla mouse Chinchillula, and the rabbit rat Reithrodon. Sigmodontines are small. They range from about 0.4 to about 14.1 oz (12 to about 400 g). The largest living form, Kunsia tomentosus, does not surpass 11.8 in (300 mm) of head and body length. External ears vary from almost absent or much reduced (e.g., the aquatic rat An-otomys and the long-clawed mole mouse Geoxus) to moderately large (e.g., the long-eared mice of the genus Phylhtis) or large (e.g., the rabbit rats of the genus Reithrodon). Climbing, arboreal and scansorial species tend to have tails longer than head and body length; tail approximately equal to combined head and body is characteristic of the more generalized terrestrial sigmodontines; while the tail of fossorial species is usually less than half of head and body length. The tail is usually thinly haired, but in some sigmodontines ends in a penciled tip (e.g., Andalgalomys). Most sigmodontines have a brownish or blackish upper pelage and grayish or whitish underparts. However, Chinchillula is noticeable in this regard; its upper parts are buffy or grayish with black lines, the underparts are white, and the white hips and white rump banded with black. Sigmodontine fur may be velvety, soft, woolly, long, thick, harsh, composed of underfur and guard hairs. Abrawayaomys, Neacomys, and Scolomys have some hairs modified into spines. Sigmodontine feet also show variation. In general, feet are relatively small, but those of fossorial forms like the giant rat Kunsia are robust and extremely long clawed; those of swimmers like the marsh rat Lundomys are large and webbed, and those of climbers (e.g., Rhipidomys) are broad and strongly grasping. Finally, the hindfoot of the arboreal mouse Rhagomys is remarkable among sigmodontine because the fifth toe is long, and the big toe presents a nail instead of a claw.

As in all rodents, one upper and one lower incisor (sometimes grooved) are found on each side of the jaw, and canines and premolars are never present. After an empty space, called diastema, following the incisor, three cheek teeth, or molars, occur on each side. The total number of teeth is 16. The exception to this pattern is the ichthyomyine Neusticomys oy-apocki that has two molars per upper and lower jaw. There is a large variation in molar size, morphology, and number of roots. For example, Reig in 1977 nominated more than 30 cusps, cuspules, styles, lophs, folds, and islands, in an idealized sigmodontine molar. Molar crown height seems to be related to diet. In general, sigmodontines that feed on animals, seeds, fruits, or fungus have low-crowned molars (brachydont) that retain complex occlusal surface. High-crowned molars (hypsodont), which extend far above the gumline, are usually present in those sigmodontines, as Andinomys and Reithrodon, which feed on abrasive vegetation (e.g., grass). Usually, the hypsodont condition is associated with a simplification of the occlusal surface, connection of lophs on each tooth side to form lamina, planation of the crown, and an increase in the number of tooth roots.

With a few well recognized exceptions, sigmodontines have complex penises. A complex penis has two lateral horns on the cartilagenous distal baculum, making it appear as trident shaped.

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